Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and improve fuel economy. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of an outlet of the EGR passage, the sensor may be sensitive to the air-fuel ratio of EGR. For example, when the engine runs with rich EGR, there is excess fuel (e.g., excess CO and H2) in the EGR. The excess fuel can react with oxygen at the sensing element of the intake oxygen sensor, reducing the oxygen concentration detected by the sensor. In addition to some of the oxygen in the air becoming equilibrated with the CO and H2 from the EGR, there is also a secondary effect due to smaller H2 molecules diffusing faster through the diffusion barrier of the oxygen sensor's sensing element, making the sensor output read richer than the true amount of excess fuel in the EGR. As another example, when the engine runs with lean EGR, the EGR contains extra air and for a given mass flow rate, the amount of real diluent in the EGR is lower. The excess oxygen in the lean EGR may be interpreted by the intake oxygen sensor as a lower EGR rate.
Matsubara teaches learning an initial calibration coefficient for the intake oxygen sensor during selected conditions when the sensor is sufficiently warm and further adjusting the calibration coefficient if the EGR is too rich or too lean. The adjusted calibration coefficient is then used to correct the output of the intake sensor.
However, the inventors have identified potential issues with such an approach. One or more other engine operating parameters are also affected by the misrepresentation of EGR by the intake oxygen sensor in the presence of rich or lean (relative to stoichiometry) EGR. For example, in the presence of lean EGR, although the sensor measures a lower (absolute) amount of EGR, the sensor output correctly reflects the burnt gas fraction. As a result, any adjustments to spark timing, throttle position, and/or fuel injection that are based on the adjusted calibration coefficient may be incorrect. As another example, in the presence of rich EGR, the sensor does not provide an accurate estimate of how much excess fuel is in the EGR. As such, if the excess fuel is not properly accounted for in cylinder fuel injection, the fuel injected will be higher than desired. This may cause open-loop fueling of the engine to be richer than desired. In the closed-loop fuel control, the adaptive fuel may adapt for the excess fuel in the EGR but the adaptive correction will be attributed to a fuel system error. This may falsely trigger a fuel system error if the correction is above a threshold. The problem may be exacerbated due to a delay between the timing of fuel injection and the sensing of the fuel at the intake oxygen sensor. Likewise, there may be delay between the EGR estimated at an exhaust oxygen sensor relative to the EGR estimated at the intake oxygen sensor. In either case, engine fueling and EGR control may be disrupted.
In one example, some of the above issues may be addressed by a method for an engine comprising: in response to an EGR air-fuel ratio being richer than a threshold, correcting an intake manifold oxygen sensor output with a correction factor based on a richness of the EGR air-fuel ratio, and adjusting each of an EGR rate and a cylinder fuel injection based on the corrected sensor output. In this way, the effect of EGR air-fuel ratio variation on the intake oxygen sensor can be accounted for and the EGR estimation and engine fueling accordingly compensated.
For example, during EGR conditions, an EGR air-fuel ratio (AFR) may be estimated by an exhaust gas oxygen sensor while an EGR rate is estimated by an intake gas oxygen sensor. If the EGR air-fuel ratio is determined to be richer than a threshold (e.g., richer than stoichiometry), a (first) correction factor may be learned based on the degree of richness of the EGR as well as an alcohol (e.g., ethanol) content of the combusted fuel. The correction factor is then applied on the output of the intake oxygen sensor to reduce the measured EGR rate based on the richness. An EGR valve may then be feedback adjusted to provide a target EGR rate. In addition, an excess fuel content of the rich EGR may be estimated based on the output of the intake sensor and the output of the exhaust sensor. A cylinder fuel injection may be adjusted based on the excess fuel content while factoring in transport delays between the exhaust sensor signal and the intake fuel injection timing.
In comparison, if the EGR air-fuel ratio is determined to be leaner than a threshold (e.g., leaner than stoichiometry), a (second) correction factor may be learned based on the degree of leanness of the EGR as well as an alcohol (e.g., ethanol) content of the combusted fuel. The correction factor is then applied on the output of the intake oxygen sensor to increase the measured EGR rate based on the leanness. An EGR valve may then be feedback adjusted to provide a target EGR rate. Alternatively, a correction factor may be applied only in the presence of rich EGR and no correction factor may be applied in the presence of lean EGR. In addition, since the intake sensor output correctly reflects the amount of burnt gas in the EGR, spark timing and airflow to the engine is adjusted based on the uncorrected sensor output.
In this way, an intake oxygen sensor output can be corrected for variations arising due to changing air-fuel ratio of EGR flowing through the sensor. By correcting the sensor output appropriately to compensate for the effects of rich or lean EGR, a more accurate dilution estimation can be provided by the sensor, improving EGR control. By inferring an amount of excess fuel in the rich EGR based on a known or calibrated composition of the rich EGR and the EGR rate determined from the corrected sensor output, and adjusting engine fuel injection in accordance, open and closed loop fuel control is improved. In addition, fewer fuel system errors may be falsely triggered. By inferring an amount of burnt gas in the lean EGR based on the uncorrected sensor output, and adjusting spark timing and intake throttle position in accordance, open and closed loop spark and air flow control is improved. Overall, corruption of sensor output when the EGR air-fuel ratio varies is reduced. By improving the accuracy of EGR dilution estimation in the presence of rich or lean EGR, engine fueling and EGR control can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.